Brayton cycle engine with high displacement rate and low vibration

Abstract

To provide refrigeration below 200 K, a Brayton cycle engine contains a light reciprocating piston. The refrigerator includes a compressor, a gas-balanced reciprocating engine having a cold rotary valve, a counterflow heat exchanger, a gas storage volume with valves that can adjust system pressures, a variable speed engine and a control system that controls gas pressure, engine speed, and the speed of the piston. The engine is connected to a load such as a cryopanel, for pumping water vapor, through insulated transfer lines.

What is claimed is: 
     
       1. A Brayton cycle engine for producing refrigeration at temperatures below 200 K, the engine comprising:
 a reciprocating cup shaped piston having a piston bottom and a cylindrical side wall, wherein said piston bottom comprises a cup bottom and a bottom cap, said bottom cap includes a material having a low thermal conductivity, said piston bottom physically and thermally separates gas in a warm displaced volume near room temperature and gas in a cold displaced volume below 200 K, said side wall sliding within a cylinder having a temperature gradient between room temperature and below 200 K, the cup shaped piston having a piston seal between the piston bottom and the cylinder, and gas flow to said cold displaced volume controlled by one of cold inlet and outlet valves, and a cold rotary valve. 
 
     
     
       2. The Brayton cycle engine in accordance with  claim 1 , wherein a length of said piston is less than a diameter of the piston. 
     
     
       3. The Brayton cycle engine in accordance with  claim 1 , wherein a thickness of said piston bottom is less than 25% of a diameter of the piston. 
     
     
       4. The Brayton cycle engine in accordance with  claim 1 , further comprising a drive stem attached to the piston bottom that is a warm side of the piston, a pneumatic force or a mechanical force acting on the drive stem to cause said piston to reciprocate. 
     
     
       5. The Brayton cycle engine in accordance with  claim 1 , wherein gas is admitted to a cold end of said piston at high pressure and exhausted to low pressure through the cold rotary valve. 
     
     
       6. The Brayton cycle engine in accordance with  claim 1 , wherein said piston reciprocates at a variable speed. 
     
     
       7. The Brayton cycle engine in accordance with  claim 1 , wherein an interior of said cylindrical side wall is at least partially evacuated. 
     
     
       8. The Brayton cycle engine in accordance with  claim 1 , wherein the piston bottom comprises at least 80% nonmetallic material. 
     
     
       9. A gas-balanced Brayton cycle engine for producing refrigeration at temperatures below 200 K, the engine comprising:
 a reciprocating cup shaped piston having a piston bottom and a cylindrical side wall, wherein said piston bottom comprises a cup bottom and a bottom cap, the cylindrical side wall is contiguous with the cup bottom, said bottom cap includes a material having a low thermal conductivity, said piston bottom physically and thermally separates gas in a warm displaced volume near room temperature and gas in a cold displaced volume below 200 K, said side wall sliding within a cylinder having a temperature gradient between room temperature and below 200 K, the cup shaped piston having a piston seal between the piston bottom and the cylinder, and gas flow to said cold displaced volume controlled by one of cold inlet and outlet valves, and a cold rotary valve; and 
 a drive stem attached to a warm side of said piston. 
 
     
     
       10. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein the cold inlet valve and the cold outlet valve admit high pressure gas when said piston is near cold end of said cylinder and exhaust gas to low pressure when said piston is near a warm end of said cylinder. 
     
     
       11. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein the cold rotary valve admits high pressure gas when said piston is near cold end of said cylinder and exhausts gas to low pressure when said piston is near a warm end of said cylinder. 
     
     
       12. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein said piston reciprocates at variable speed. 
     
     
       13. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein a double bumper is actuated by said drive stem. 
     
     
       14. The Brayton cycle engine in accordance with  claim 1 , wherein the bottom cap and the cup bottom are made of different materials. 
     
     
       15. The Brayton cycle engine in accordance with  claim 1 , wherein the cup bottom is metallic. 
     
     
       16. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein the bottom cap and the cup bottom are made of different materials. 
     
     
       17. The gas-balanced Brayton cycle engine in accordance with  claim 9 , wherein the cup bottom is metallic.